The present invention relates to a gas sensor, and particularly to a combined gas sensor for detecting the concentration of acid gases such as nitrogen oxides, etc. in exhaust gases and oxygen concentration in a gas atmosphere to be detected.
Various poison gases including nitrogen oxides (NOx), hydrocarbon gases (HC), sulfur oxides (SOx), CO, CO2, and etc., are exhausted from internal combustion engines including those of automobiles, and combustion instruments of thermal power plants, and factory plants, etc. They are called environmental gases, and severe legal regulations have been enforced to limit the emission of environmental gases. Thus, the development of a gas sensor capable of detecting the low concentrations of the exhausted environmental gases at a low cost has been demanded.
Recently, an all-solid-type gas sensor which can carry out the continuous measurement by direct insertion in exhaust gases from engines of automobiles, etc., has been watched with keen interest, and many investigations and developments thereof have been reported. The present inventors already proposed a mixed potential-type NOx sensor capable of measuring the total NOx concentrations in exhaust gases at real time. For example, in Japanese Patent Laid-Open Publication No. 201942/1999, a mixed potential-type NOx sensing electrode is disposed in a gas chamber formed by Zirconia solid electrolyte, and also a NOx converting electrode is disposed in the same chamber such that it faces to the NOx sensing electrode. That is, in this structure, NOx (NO and NO2) in exhaust gases are electrochemically converted to a NO2 single substance gas, and the converted NO2 concentration is detected by the NOx sensing electrode as the total NOx concentration. In the embodiment of Japanese Patent Laid-Open Publication No. 201942/1999, an oxygen pump (converting pump) for converting NOx in the gas chamber to NO2 and another oxygen pump for controlling the oxygen concentration in a gas chamber are disposed. Also, a reference electrode to the NOx sensing electrode described above and an oxygen sensing electrode for measuring the oxygen concentration in the gas chamber are disposed in the same gas chamber. By referring the potential of the oxygen sensing electrode due to the oxygen concentration in the gas chamber as a reference potential of the NOx sensing electrode, the output of the NOx sensor is hardly influenced even when the oxygen concentration in the gas chamber is fluctuated.
The fluctuation of the oxygen concentration is, however, large and sharp in the exhaust gases from e.g. an automobile. Therefore, in the NOx sensor element with the gas chamber structure shown in Japanese Patent Laid-Open Publication No. 201942/1999, etc., it is necessary to make the oxygen concentration in the gas chamber at a level at which interference gases such as HC, CO, etc., entering the gas chamber together with NOx, are oxidized to non-interference gases. The oxygen concentration around the sensing electrode based on mixed potential should also be sustained at a substantially constant value of several percents. Accordingly, the occurrence of disorder of a system, in which the detection of the oxygen concentration in a gas chamber is carried out and the oxygen pump driving is controlled by feeding back the signal of the oxygen concentration, gives a large error to the accuracy of the sensor output. Also, as a precondition, it is necessary that the gas tightness in the gas chamber and the air duct is kept in order.
On the other hand, the temperature of the exhaust gases from automobiles is increasing more and more recently, and the gas temperature changes largely, depending on the driving states of engine thereof. Also, poison components for the electrodes are largely contained in exhaust gases, and further, the vibration of the body or the engine of automobiles is large. Thus, an improvement of the reliability of the gas sensor used in exhaust gases under such severe circumstances becomes very important.
However, in the conventional NOx sensor disclosed in Japanese Patent Laid-Open Publication No. 201942/1999, even when disorder occurs in the system in which the detection of the oxygen concentration in the gas chamber is carried out, and the oxygen pump driving is carried out by feed-back control, it was impossible to self-diagnosed diagnose whether the sensor was in order or not. Also, when the gas tightness of the gas chamber and the air duct, is lost during the use to influence the sensor output, the NOx sensor did not have any means for informing a user whether or not such is due to the sensor disorder. That is, the above-described NOx sensor has problems that when deterioration of electrodes, disorder in controlling the oxygen concentration in the gas chamber, and losing of the gas tightness of the gas chamber or the air duct occur during the use of the gas sensor, it cannot be determined that they are due to disorder of the sensor or not.
The present invention has been made for solving the above-described problems.
According to the invention, there is provided a combined gas sensor having a structure equipped with a first gas chamber formed by an oxygen ion conductive solid electrolyte; a first gas inlet having a diffusion resistance for restricting the flow amount of gases entering the first gas chamber; a first oxygen sensing electrode active only to oxygen, which is disposed on a surface of the first electrolyte substrate inside the first gas chamber; a gas sensing electrode active to at least target gases and oxygen, which is disposed on the same surface of the first electrolyte as the first oxygen sensing electrode; a reference electrode active to oxygen, which is disposed on the first solid electrolyte substrate such that it is brought into contact with an atmospheric air separated from the first gas chamber; an oxygen pump cell for controlling oxygen concentration in the first gas chamber, of which electrodes are made of materials active only to oxygen and disposed on a second solid electrolyte substrate facing to the first solid electrolyte substrate; a gas reforming pump cell for chemically or electrochemically converting or decomposing the gases, of which a gas reforming electrode is active to oxygen and the target gases, and disposed on the second solid electrolyte substrate in the first gas chamber so as to face to the gas sensing electrode, and a gas reforming counter electrode is disposed on the second solid electrolyte substrate outside the first gas chamber such that it is brought into contact with the atmospheric air; and a second oxygen sensing electrode active only to oxygen, which is disposed on the first solid electrolyte substrate such that it is directly exposed to the detection gas atmosphere.
Furthermore, according to the invention, there is provided a combined gas sensor comprising an oxygen pump cell for controlling the oxygen concentration in the first gas chamber, of which the electrodes are made of materials active only to oxygen and disposed on the second solid electrolyte substrate facing to the first solid electrolyte substrate; and a gas reforming pump cell for chemically or electrochemically converting or decomposing the gases, of which a gas reforming electrode is active to oxygen and the target gases and disposed on the second solid electrolyte substrate in the first gas chamber so as to face to the gas sensing electrode, and a gas reforming counter electrode is disposed on the second solid electrolyte substrate outside the first gas chamber such that it is brought into contact with the atmospheric air, and characterized by having driving cell circuits wherein a DC variable power supply for controlling the oxygen concentration in the first gas chamber at a constant level by feed-back control of pumping currents for the oxygen pump cell, according to an electromotive force between the oxygen sensing electrode and the reference electrode as a signal output of the oxygen concentration in the first gas chamber, and a DC constant voltage power supply for making oxygen pumping currents of the gas reforming pump cell constant, and thus making an electrochemical decomposition or conversion amount of the target gas constant; are connected to the oxygen pump cell and the gas reforming pump cell, respectively.
According to the invention, in the gas sensor having the gas chamber formed by the oxygen ion conductive solid electrolyte, the oxygen concentration in the gas chamber or the oxygen pumping current is referred to the oxygen concentration in the detection gas atmosphere in order for comparison and judgment, whereby the disorder of the gas sensor element can be self-diagnosed. Thus, the operation state of the gas sensor can be always automatically monitored. In particular, when the invention is applied to a gas sensor for mounting on an automobile with severe environment, the reliability of the sensor is greatly improved. Furthermore, because according to the gas sensor of the invention, the oxygen concentration in the detection gas atmosphere can be directly measured, when the gas sensor is, for example, a NOx sensor for mounting on an automobile, the concentration of NOx and oxygen in the exhaust gases can be simultaneously measured.